This invention relates to non-linear optical processes in gas phases and, more particularly, to a unidirectional light amplifier using a plurality of lasers.
With the advances of non-linear optical processes in gas phases it is desirable to study ultrafast coherent transient spectroscopic techniques to probe the basic physics of non-linear interactions of light with matter. It is particularly useful to use the light shift induced by the interaction of atoms with intense monochromatic light which can be used to compensate for Doppler broadening. It is usually preferable to implement high intensity lasers using gaseous media instead of solid media in order to avoid destructive thermal stress on solid materials used in such systems. However, the atoms of a gaseous medium experience thermal motion due to their not being locked into a relatively rigid lattice structure. Consequently, if excited gaseous atoms emit light, the photons so emitted have a range of thermal velocities which causes a Doppler shift (i.e. change in frequency of the photons or radiation) along the axis of observation. This effect gives rise to Doppler broadening of the radiation with the result that the energy is spread over a larger bandwidth of frequencies of the photons. However, if the excited gaseous atoms are subjected to monochromatic light of appropriate wavelength from a laser, the energy levels of the excited atoms may be shifted as a result of Stark effect due to the electromagnetic field of the impinging radiation from the laser. Such shifts in the energy levels are velocity-dependent and can be used to obtain Doppler free radiation. CohenTannoudji et al first demonstrated theoretically in their paper entitled "Compensating Doppler Broadening with Light Shifts", Optics Communications, Vol. 27, pp. 71-75 (October 1978) that an external laser could be used to compensate for Doppler shift of the emitted radiation from excited atoms by arranging the velocity-dependent light-induced effect to be complementary to Doppler broadening and thus the two effects cancelling each other. Reynaud et al in their paper entitled "Compensation of Doppler Broadening by a Velocity-Dependent Light-Shifts", Laser Spectroscopy, Vol. 4 (1979) pp. 96-105 discuss on pages 98, 104, observation of Doppler-free spectral lines in an experiment on neon and thus demonstrated forward/backward asymmetry for the emitted radiation. However, the true unidirectionality of the emitted radiation and its amplification characteristic beyond a threshold was not experimentally demonstrated and it was considered desirable to explore these aspects. It is thus desirable to have a light-induced unidirectional light amplifier system which can have wide applications.